Variation in susceptibility of threatened flora to Phytophthora cinnamomi

Determination of susceptibility to Phytophthora cinnamomi affirms the exceptional broad host range of the pathogen and diversity of host-pathogen interactions in the native flora of the South-West Botanical Province of Western Australia. Positioning taxa on the P. cinnamomi resistance-susceptibility continuum prioritises taxa in relation to the threat posed by the pathogen. Variation in susceptibility of 501 Western Australian plant taxa to P. cinnamomi infection was evaluated after soil inoculation. Disease progress curves following soil inoculation were analysed with the three logistic model parameters of upper asymptote (K-max), lag time (t(A1/2K)) and intrinsic rate of increase (r). The susceptibility continuum represented by the ordination of parameters of the logistic model for % plant mortality in two dimensions was similar for threatened, priority and common taxa. Variation in parameters of the logistic model decreased with susceptibility. Although resistant taxa had low K-max, the values for delay and rate varied. In comparison, susceptible taxa tended to have high K-max short delay and fast rate. Most of the taxa in Fabaceae, Malvaceae, Myrtaceae and Poaceae tested were resistant to P. cinnamomi. A few taxa in Fabaceae and Myrtaceae (18 and 10 %, respectively) were susceptible to the pathogen. Casuarinaceae were mainly resistant, but showed a range of responses to P. cinnamomi. Ericaceae were mainly susceptible. Proteaceae showed a broad range of responses from resistant to susceptible. Significantly greater resistance of sprouters than seeders and the significant negative relationship between mortality and leaf phenolics, may be examples of resistance mechanisms through fortuitous side effects of ecological fitting of functional traits that evolved in response to different sets of biotic and abiotic conditions that occurred before the introduction of P. cinnamomi. Identification of resistant individuals in threatened taxa would ensure survival of taxa and conservation of genetic diversity.